Positron Emission Tomography (PET) is an important medical imaging technique which provides an image of the distribution of a positron emitting radio pharmaceutical in the body on a metabolic level. At the present time, the performance of this important instrument is limited by the sensors used. Future advances in PET system technology can only come about by performing research to improve nuclear sensors. This effort will examine an approach to improve the resolution of a PET system by using two types of sensors. A photomultiplier tube (PMT) is the primary detector. In addition, one or more compact photodetectors are used to obtain better spatial resolution. The goal of Phase I of this program is to demonstrate the feasibility of using T1BrI photodetectors as the semiconductor photosensor to make this approach work. If successful, the use of T1BrI photodetectors in this dual-sensor approach promises to be the breakthrough needed for the construction of a new generation of high resolution PET imaging systems.